The flyback converter is used in both AC/DC and DC/DC conversion with galvanic isolation between the input and output. The flyback converter is a buck-boost converter with the inductor split to form a transformer, so that the voltage ratios are multiplied with an additional advantage of isolation. A switch is used to enable energy transfer from the input voltage source to the transformer. When the switch is closed, the primary side of the transformer is directly connected to the input voltage source. The primary side current and magnetic flux in the transformer increases, storing energy in the transformer. The corresponding voltage induced in the secondary side is negative, so the diode connecting the secondary side to the load is reverse-biased. The output capacitor supplies energy to the output load. When the switch is opened, the primary side current and magnetic flux drops. The secondary side voltage becomes positive, forward-biasing the diode and allowing current to flow from the transformer. The energy from the transformer core recharges an output capacitor and supplies the load.
A very high inductive kickback (reflected voltage) occurs when the switch turns off, placing a great deal of voltage strain on the transistor. This voltage strain can be reduced by using two transistors in the flyback converter. Doing so can prevent the voltage across either switch from exceeding the input voltage level. Conventional two-transistor flyback converters have an RCD (resistor-capacitor-diode) network in the diode return path from the source of the high-side transistor to ground. The high-side transistor is switchable for connecting the input voltage source to the primary side of the transformer. The capacitor of the RCD network charges to the difference between the transformer reflected voltage and the input voltage, and prevents current flow from the transformer back to the input source. However, additional switching losses occur due to the charging and discharging of the capacitor that is part of the RCD network. Also, the voltage on the high-side transistor exceeds the input voltage. When such a topology is used as a power factor correction unit, the input voltage changes from zero to a certain voltage and power loss created by the capacitor of the RCD network becomes substantial.